1. Field of the Invention
The present invention relates to an optical system supporting device. More particularly, the present invention is concerned with an optical system supporting device which can suitably be used as means for supporting an objective lens in an optical disc recording/reproducing apparatus.
2. Discussion of Related Art
An optical system supporting device for use in an optical disc recording/reproducing apparatus is, for example, disclosed in Japanese Patent Application Laid-open Specification No. 197519/1994. FIG. 3 as attached hereto is a perspective view showing the entire construction of an actuator employed therein, FIG. 4 is an exploded perspective view of the actuator, and FIG. 5 is another exploded view corresponding to that of FIG. 4 viewed upside down in the Z-direction of FIG. 4.
A lens holder 2 is fitted at its center with an objective lens 1. Both Y-direction ends of the lens holder 2 are provided with respective recesses 2a, 2a. Focus coils 3, 3 having a winding made in the form of a square pole are arranged in these recesses 2a, 2a. The recesses 2a, 2a comprise respective bottom faces 2b, side faces 2c and parts 2d protruding upward from both X-direction ends of the bottom face 2b. The focus coils 3, 3 have their bottom faces positioned and fastened by means of the bottom faces 2b of the recesses 2a, 2a, have their peripheral surfaces positioned and fastened by means of the side faces 2c of the recesses 2a, 2a and have their inside faces positioned and fastened by means of the protrudent parts 2d of the recesses 2a, 2a.
The focus coils 3, 3 are fitted at their remaining peripheral surfaces with tracking coils 4, 4. These tracking coils 4, 4 are arranged in positions oppositely deviated in the X-direction. The protrudent parts 2d are brought into abuttal with X-direction side faces of below described yokes 14 and thus act as X-direction stoppers for the lens holder 2.
The lens holder 2 is fitted at its bottom face with a balancer 5 formed so as to have a shape along the periphery of the lens holder 2. This balancer 5 is made of a phosphor bronze plate. Referring to FIG. 5, a side of the lens holder 2 and a side of the balancer 5 are respectively provided with circular-arc-shaped protrudent parts 2e, 5a. Light beam emitted from a fixed optical system (not shown) passes just under this protrudent part 5a and falls incident upon a mirror (not shown). The balancer 5 is secured substantially along the periphery of the bottom face of the lens holder 2 to thereby increase the torsional rigidity around X-axis of the lens holder 2 and accordingly cause an increase of resonance frequency. Also, the protrudent part 2e formed in the lens holder 2 acts so as to increase the rigidity of the lens holder 2. A square recess part 2j for disposing a mirror is formed on the bottom side of the lens holder 2 so that the objective lens 1 is positioned in the vicinity of the mirror.
Both X-direction ends of the lens holder 2 are provided with protrudent parts 2f, 2f. Grooves 2g, 2g are formed on the upper and lower sides of the protrudent parts 2f, 2f. One-side ends 6a, 6a, 7a, 7a of springs 6, 6, 7, 7 are respectively secured to these grooves. Further, these spring ends 6a, 6a, 7a, 7a are directly soldered to terminals (not shown) of the focus coils 3 and the tracking coils 4. Other-side ends 6b, 6b, 7b, 7b of the springs 6, 6, 7, 7 are respectively secured with solder to grooves 8a, 8a, 8b, 8b provided at both X-direction ends of a spring fixing member 8. These grooves 8a, 8a, 8b, 8b of the spring fixing member 8 composed of a plastic and a spring fixing part 8f disposed on the back side of the spring fixing member 8 are provided with copper patterns to thereby enable electric connection to the springs 6, 6, 7, 7. Thus, FPC is not required therein, so that the problem can be obviated that the springs suffer from stresses due to, for example, a deformation of FPC. The springs 6, 6, 7, 7 are produced by etching a beryllium bronze plate and partially coated with dampers 9, 9, 9, 9. These springs 6, 6, 7, 7 holds the lens holder 2 in such a manner that the lens holder 2 can move in the X- and Z-directions. These springs 6, 6, 7, 7 are bent inward in the X-direction, and the spacings between bilaterally arranged springs 6, 7 are so set that the spacings between ends 6b, 6b and 7b, 7b on the side of the spring fixing member 8 are larger than the spacings between ends 6a, 6a and 7a, 7a on the side of the lens holder 2. This arrangement enables reducing the X-direction size on the side of the lens holder 2 and enables increasing the torsional rigidity around Y-axis.
Two bosses (not shown) are formed on the bottom face of the spring fixing member 8. The spring fixing member 8 is positioned on a base 10 by engaging the bosses with holes 10a, 10a made in the base. A housing 8d is integrated through a connecting part 8c with the spring fixing member 8. The housing 8d is provided at its one end with a resin injection gate 8g. The housing 8d is further provided with a recess 8e, in which a flag 2h disposed at one front end of the protrudent part 2f of the lens holder 2 is placed when the actuator 22 is assembled. LED 11 and PD 12 are fitted in the housing 8d. A light receiving face of the PD 12 are halved in the X-direction. Light emitted in the Y-direction from the LED 11 has its central zone shielded by the above flag 2h and falls incident upon the PD 12, so that the shadow of the flag 2h appears on a halving line of the PD 12. Therefore, the flag 2h, namely, information on the position of the objective lens 1 in the X-direction and information on the moving velocity thereof can be recognized by gaining an output difference of the light receiving face of the PD 12.
The connecting part 8c of the spring fixing member 8 is placed between the upper and lower springs 6, 7, so that the outline of the actuator 22 is not enlarged by the connecting part 8c.
The springs 6, 6, 7, 7 are assembled while the lens holder 2 and the spring fixing member 8 are positioned with the use of respective jigs. The flag 2h of the lens holder 2 is formed integrally with the lens holder 2 and the housing 8d is also formed integrally with the spring fixing member 8, so that the accuracy of mutual positional relationship of the LED 11, PD 12 and flag 2h is desirable and that an offset of differential output of the PD 12 is less probable. This construction renders unnecessary the position regulating workload at assembly. That is, it is intended to attain the reduction of part and assemble costs through the integration of a plurality of parts having been employed and through the avoidance of regulating work therefor. Moreover, the position sensing parts (flag 2h, LED 11, PD 12) for the objective lens 1 are placed opposite to the fixed optical system with respect to the mirror, so that the light beam between the mirror and the fixed optical system is not interfered with to thereby facilitate the arrangement of the position sensing parts.
The base 10 is formed by conducting a press molding of an iron plate. Four yokes 13, 13, 14, 14 extending in the X-direction are projecting in the Z-direction. Magnets 15, 16 magnetized in the direction of thickness are secured with the N poles inside to the inside faces of the outer yokes 13, 13. Magnetic gaps are formed between the magnets 15, 16 and the inner yokes 14, 14. Each of the outer yokes 13, 13 has its one X-direction end bent inward to thereby form a yoke 13a. Referring to FIG. 3, upon the assembly of the actuator 22, one side 4a, 4a of the tracking coil 4, 4 is placed opposite to the magnet 15, 16 and the other side 4b, 4b of the tracking coil 4, 4 is placed opposite to the yoke 13a, 13a. The direction of magnetic flux in the above magnetic gap at the one side 4a of the tracking coil 4 is opposite in the Y-direction to that at the other side 4b of the tracking coil 4, so that the directions of forces occurring at these sides 4a, 4b are the same in the X-direction. Further, one side of the focus coil 3 is also placed in this magnetic gap, so that a force occurs in the Z-direction.
FPC 15 is bonded to a lower face of the base 10 and soldered to LED 11 and PD 12 terminals passing through holes 10b, 10b made in the base 10 to thereby attain an electrical connection of these elements. One X-direction end of the base 10 is bent vertically upward to thereby form a reinforcing part 10c. The other X-direction end of the base 10 has its lower side cut off in the form of a circular arc to thereby form a light beam escape 10d. Light emitted from the fixed optical system passes just under this light beam escape 10d and falls incident upon the mirror.
It is required that the above optical system supporting device be capable of accurately supporting the objective lens without causing the objective lens to shift from the appropriate location or suffer from an inclination even in an atmosphere of low temperature such as 0.degree. C. to high temperature such as 60.degree. C. Especially, in recent years, the optical disc is oriented toward a reduced track width and an increased data density for higher capacity, so that stricter restriction is posed on the shift and inclination of objective lens which invite a light spot aberration on a medium surface. For example, the influence of the inclination of the objective lens on the window margin is as shown in FIG. 6. The window margin is a property on which the recording and reproducing performance of the optical disc recording and reproducing apparatus can be evaluated. The window margin should be at least 30% for ensuring desirable recording and reproduction of information. When the inclination of the objective lens is nearly zero, the window margin is substantially constant. However, when the inclination of the objective lens exceeds about .+-.0.2.degree. (.+-.12'), the window margin sharply drops to thereby render the recording and reproduction of information difficult.
The spring fixing member 8 of the prior art has its parts integrated together for cost reduction so as to assume a laterally nonsymmetrical complex configuration of small size and small thickness and is obtained by injection molding. The plastic injected through the gate 8g disposed at one end of the housing 8d first fills the thin housing 8d and connecting part 8c and then flows therefrom so as to fill the spring fixing part 8f. Therefore, because the plastic flows thereinto from a position of about 1/4 of the spring fixing part 8f, the plastic orientation (flow) is laterally nonsymmetrical with respect to the spring fixing part 8f. Moreover, because the plastic at the time of the above flow has a low temperature to thereby have a low fluidity, the plastic orientation is not smooth and an unbalanced stress remains in a solidified plastic.
Generally, the plastic is a high molecular polymer and has properties which are different according to the direction of alignment of the polymer molecule. The plastic exhibits the so termed anisotropy such that there is a difference between the coefficient of linear expansion along the direction of flow at the time of molding and the coefficient of linear expansion along the direction perpendicular to the above flow direction. The plastic for use in the optical system supporting device is compounded with 20 to 40% of a fibrous reinforcing material such as glass fiber for increasing the strength and rigidity thereof, and the direction of orientation of this compounded reinforcing material increases the anisotropy of the coefficient of linear expansion. Liquid crystal polymers (LCP) of desirable fluidity are generally used with an emphasis placed upon a small-thickness moldability as the plastic material of the spring fixing member 8. However, the liquid crystal polymers are materials of high anisotropy and, according to catalog data, the coefficient of linear expansion thereof is 0.7.times.10.sup.-5 /.degree.C. along the flow direction and 4.4.times.10.sup.-5 /.degree.C. along the direction perpendicular thereto. The ratio of these values is 1:6.28.
When the spring fixing member 8 formed from the material of nonsymmetrical plastic orientation having residual stress and high anisotropy encounters a temperature change, the spring fixing member 8 cannot uniformly expand and suffers from deformation such as warpage and torsion. In the actual measurement, it has been found that right and left faces of the groove 8a of the spring fixing member 8 incline as much as about 5' (0.083.degree.) each in opposite directions by a temperature rise of 30.degree. C.
Consequently, the four springs 6, 6, 7, 7 secured to the grooves 8a, 8b of the spring fixing member 8 move in directions which are different from each other. Further, even if the deformation of the employed spring fixing member 8 is slight, the front edge portions of the springs 6, 7 suffer from extended deformation. For example, providing that the grooves 8a, 8b of the spring fixing member 8 have their faces inclined as much as 5' (0.083.degree.) and that each of the springs 6, 7 has a length of 10 mm, the deformation of the front edge portion of each spring is about 15 .mu.m (=10.times.10.sup.3 sin 0.083.degree.), which is as large as the sum of widths of at least 10 optical disc tracks. When the upper springs 6, 6 and the lower springs 7, 7 shift from each other in opposite directions as much as the above 15 .mu.m, the lens holder 2, namely, the objective lens 1 is inclined as much as about 41' (0.69.degree.)(=sin.sup.-1 (2.times.15/2.5.times.10.sup.3)) because the upper and lower springs are spaced from each other as much as about 2.5 mm. This inclination means an 8-fold expansion of the deformation of the spring fixing member 8. An application of this inclination of the objective lens to the graph of FIG. 6 suggests that the window margin is nearly 0. Thus, it is apparent that the objective lens is of no practical use.
Therefore, the prior art has a drawback in that the objective lens 1 is so inclined that the laser spot focusing on an optical disc medium surface suffers from an aberration to thereby disenable accurate recording of information on the optical disc and reading of recorded information from the optical disc.